<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

</head>
<body>
    <input type="radio" name="sex" value="1">
    <input type="radio" name="sex" checked value="2">
    <input type="radio" name="sex" value="3">
    <input type="checkbox" value="1">
    <input type="checkbox" value="2">
    <input type="checkbox" value="3" checked>
    <select name="" id="">
        <option value="1">1</option>
        <option value="2">2</option>
        <option value="3" selected>3</option>
    </select>
    <script src="jquery-1.12.2.min.js"></script>
    <script>
       $(function(){
           console.log($(":checked"));          //单选、复选、下拉框 option
           console.log($("input:checked"));          //单选、复选
       });
    </script>
</body>
</html>